You can significantly increase your recall rate by searching on alternative medieval spellings for your target. For example, instead of
Seneschal OR senechal OR senescal OR senescall OR senescallum
Hainault OR Hainaut OR Haynau OR Hynnault OR Henaud OR Henaude
I've started building a thesaurus file of alternate spellings of names and words of interest to me. I can paste them into the Google search panel as needed. If you want words that aren't proper names, the Middle English Dictionary is a good resource, and there are middle French dictionaries online as well.